Surface acoustic wave element

ABSTRACT

A highly reliable surface acoustic wave device includes a low-resistance piezoelectric substrate that is difficult to reoxidize even in a high-temperature atmosphere containing oxygen. The piezoelectric substrate has a specific resistance of about 1.0×10 7  Ω·cm to about 1.0×10 13  Ω·cm, an interdigital electrode on a main surface of the substrate, and a protective film covering the interdigital electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surface acoustic wave elements for usein, for example, surface acoustic wave filters or the like, and morespecifically, the present invention relates to a surface acoustic waveelement that has a structure that prevents electrodes from being brokenby the pyroelectric effect of the piezoelectric substrate.

2. Description of the Related Art

In a surface acoustic wave element, electrodes connected to differentpotentials of the interdigital transducer (IDT) may cause electricdischarge due to: (a) electrical charges on the surface of thepiezoelectric substrate resulting from the pyroelectric effect of thesubstrate produced by temperature changes; or (b) electrical chargesinduced by applying a surge voltage. If the amount of electric dischargeis large, the electrodes are degraded or broken. Accordingly, thecharacteristics of the surface acoustic wave element are deteriorated.

In order to prevent the electric discharge from damaging the electrode,the specific resistance of the piezoelectric substrate can be reduced. Areduced specific resistance of the substrate allows electrical chargessuddenly produced on the surface of the substrate to move on the surfaceof the substrate to reduce the potential difference rapidly. Thus,electric discharge resulting from local charge accumulation can beprevented. Consequently, the resistances to pyroelectric destruction andto electric power can be enhanced in a surface acoustic wave deviceusing a piezoelectric substrate, such as a surface acoustic wave filter.

Some approaches for reducing the specific resistance of the substratehave been known in which the substrate may be doped with Fe or othercarriers from the surface, or the substrate may be heated in a reducingatmosphere of a specific gas under reduced pressure.

For example, Japanese Unexamined Patent Application Publication No.11-92147 has disclosed LiNbO₃ and LiTaO₃ crystals preconditioned so asto increase the ability to reduce surface charging and a method forpreparing those crystals. More specifically, Japanese Unexamined PatentApplication Publication No. 11-92147 has disclosed a method ofheat-treating a LiNbO₃ or LiTaO₃ crystal in a reducing atmosphere at atemperature of 500° C. to 1,140° C. under reduced pressure. However,this publication has not described specific conditions of the reducedpressure in the reducing atmosphere. According to this method, a gas,such as argon, water, hydrogen, nitrogen, carbon dioxide, carbonmonoxide, oxygen, or a mixture thereof is used for preparing thereducing atmosphere.

In general, in the manufacturing process of a surface acoustic wavedevice, some steps are performed in a high-temperature atmospherecontaining oxygen. For example, in a manufacturing process of achip-size-packaged surface acoustic device in which a surface acousticwave element is mounted on a mount board by flip chip bonding and sealedwith a resin, the step of the flip chip bonding and the step ofthermally curing the resin are performed in high-temperature atmospherescontaining oxygen. In a manufacturing process of a surface acoustic wavedevice in which a surface acoustic wave element is mounted on a ceramicpackage by flip chip bonding, the step of the flip chip bonding isperformed in a high-temperature atmosphere containing oxygen. In amanufacturing process of a surface acoustic wave device including thestep of mounting a surface acoustic wave element to a ceramic package bywire-bonding after die bonding, the step of thermally curing a diebonding agent is performed in a high-temperature atmosphere containingoxygen.

Although the specific resistance of the piezoelectric substrate used forthe surface acoustic wave element is reduced by heat treatment underreduced pressure in a reducing atmosphere, as disclosed in the JapaneseUnexamined Patent Application Publication No. 11-92147, thepiezoelectric substrate is reoxidized if the surface acoustic waveelement is exposed to a high-temperature atmosphere containing oxygen inthe above-described manufacturing processes of the surface acoustic wavedevice. In the resulting surface acoustic wave device, therefore, thepiezoelectric substrate is liable to experience the pyroelectric effectagain, in spite of using the method of Japanese Unexamined PatentApplication Publication No. 11-92147. Consequently, electric dischargeoccurs between electrodes connected to different potentials of the IDT,and degrades or damages the electrodes disadvantageously.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a highly reliable surface acoustic waveelement including a piezoelectric substrate with a reduced specificresistance. The surface acoustic wave element does not allow thepyroelectric effect to reproduce, and accordingly, its electrodes arenot easily degraded or broken.

According to a first preferred embodiment of the present invention, asurface acoustic wave element includes a piezoelectric substrate havinga specific resistance in a range of about 1.0×10⁷ Ω·cm to about 1.0×10¹³Ω·cm, an interdigital electrode disposed on one of the main surfaces ofthe piezoelectric substrate, and a protective film covering theinterdigital electrode.

According to a second preferred embodiment of the present invention, asurface acoustic wave element includes a piezoelectric substrate havinga specific resistance in a range of about 1.0×10⁷ Ω·cm to about 1.0×10¹³Ω·cm, an interdigital electrode overlying one of the main surfaces ofthe piezoelectric substrate, and a protective film underlying theinterdigital electrode.

In the surface acoustic wave element according to the second preferredembodiment, a second protective film may further be arranged so as tocover the interdigital electrode.

In preferred embodiments of the present invention, the protective filmmay preferably be made of a material selected from the group consistingof SiN, ZnO, and SiO₂.

The protective film may preferably include a SiN or ZnO layer and a SiO₂layer deposited on the SiN or ZnO layer.

The piezoelectric substrate may preferably be made of LiTaO₃ or LiNbO₃.

The surface acoustic wave device may further include an electrode padfor establishing external electrical connection, disposed on thepiezoelectric substrate. In this instance, the protective film isarranged so as to have a region at which the electrode pad is exposed,and the region has an area smaller than the area of the electrode pad.

Other features, elements, characteristics and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments thereof with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic front sectional view of a surface acoustic wavedevice according to a first preferred embodiment of the presentinvention.

FIG. 1B is a schematic plan view of the structure of electrodes of asurface acoustic wave element used in the surface acoustic wave deviceof FIG. 1A.

FIG. 2 is a plot of voltages induced at the surfaces of piezoelectricsubstrates with a low specific resistance by heating the substrates tovarious temperatures for an hour in air or a nitrogen atmosphere.

FIG. 3A is a schematic front sectional view of a surface acoustic wavedevice according to a second preferred embodiment of the presentinvention.

FIG. 3B is a bottom view of a surface acoustic wave element included inthe surface acoustic wave device of FIG. 3A.

FIG. 4 is a schematic front sectional view of a surface acoustic wavedevice according to a third preferred embodiment of the presentinvention.

FIG. 5 is a schematic front sectional view of a surface acoustic wavedevice according to a fourth preferred embodiment of the presentinvention.

FIG. 6 is a schematic front sectional view of a surface acoustic wavedevice according to a fifth preferred embodiment of the presentinvention.

FIG. 7 is a schematic front sectional view of a surface acoustic wavedevice according to a sixth preferred embodiment of the presentinvention.

FIG. 8 is a schematic front sectional view of a surface acoustic wavedevice according to a seventh preferred embodiment of the presentinvention.

FIG. 9 is a schematic front sectional view of a modification of thesurface acoustic wave device according to the third preferredembodiment.

FIG. 10 is a schematic front sectional view of a modification of thesurface acoustic wave device according to the fifth preferredembodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the drawings.

FIG. 1A and FIG. 1B are a schematic front sectional view of a surfaceacoustic wave device according to a first preferred embodiment of thepresent invention and a schematic plan view of the structure ofelectrodes of a surface acoustic wave element used in the surfaceacoustic wave device, respectively.

A surface acoustic wave device 1 includes a surface acoustic waveelement 3 mounted on a mount board 2 preferably by face-down bonding.The surface acoustic wave element 3 is covered with a sealing resin 4.

The surface acoustic wave element 3 includes a piezoelectric substrate5. As shown in FIG. 1A, interdigital electrodes 6 and electrode pads 7and 8 are disposed on one main surface 5 a of the piezoelectricsubstrate 5. The electrode pads 7 and 8 are electrically connected tocomb-like electrodes 6 b and 6 a of the interdigital electrodes 6,respectively.

Although FIG. 1A shows only the interdigital electrodes 6 and theelectrode pads 7 and 8 in the surface acoustic wave element 3 for easeof understanding the structure, the surface acoustic wave element 3 ofthe present preferred embodiment has the structure shown in FIG. 1B.Specifically, a single port SAW resonator R and a longitudinally coupledresonator type surface acoustic wave filter F capable ofbalanced-to-unbalanced conversion are disposed on the piezoelectricsubstrate 5, as shown in FIG. 1B. The single port SAW resonator Rincludes an interdigital electrode 6 a and a pair of reflectors disposedon both sides of the interdigital electrode 6 a. The longitudinallycoupled resonator type surface acoustic wave filter F preferablyincludes three interdigital electrodes 6 b to 6 d and a pair ofreflectors disposed at both ends of the line of the interdigitalelectrodes 6 b to 6 d, for example.

As shown in FIG. 1B, the interdigital electrodes 6 a and 6 b to 6 d andthe other electrodes constituting the surface acoustic wave resonator Rand longitudinally coupled resonator type surface acoustic wave filter Fare coated with a protective film 9. In addition to the electrode pads 7and 8, electrode pads E1 to E3 connected to the ground potential arealso provided on the main surface 5 a of the piezoelectric substrate 5.For ease of understanding the function of the protective film 9, FIG. 1Aschematically shows the structure of the electrodes.

The piezoelectric substrate 5 is preferably made of a specially treatedpiezoelectric single crystal, such as LiTaO₃ or LiNbO₃. Thepiezoelectric substrate 5 preferably has a specific resistance of about1.0×10⁷ Ω·cm to about 1.0×10³ Ω·cm in the thickness direction. Apiezoelectric substrate with a specific resistance of more than about1.0×10¹³ Ω·cm produces the pyroelectric effect, and is liable to degradeor brake the interdigital electrodes if its temperature is changed. Apiezoelectric substrate with a specific resistance of less than about1.0×10⁷ Ω·cm has low piezoelectric, and accordingly exhibitscharacteristics sufficient for a surface acoustic wave element.

This level of the specific resistance can be achieved by reducing apiezoelectric single crystal substrate with oxygen.

A piezoelectric substrate 5 having a specific resistance of about1.0×10¹³ Ω·cm or less hardly has the pyroelectric effect.

The interdigital electrodes 6 and the electrode pads 7 and 8 arepreferably made of an elemental metal, such as Cu or Al, or its alloy.

The surface acoustic wave element 3 is mounted on the mount board 2 suchthat its main surface 5 a faces downward.

More specifically, electrode pads 11 and 12 are provided on the uppersurface 2 a of the mount board 2, and the electrode pads 11 and 12 arebonded to the electrode pads 7 and 8 of the surface acoustic waveelement 3 preferably via metal bumps 13 and 14, respectively. The metalbumps 13 and 14 may be formed of an appropriate metal material, such asan elemental metal or solder.

The mount board 2 has connecting electrodes 15 and 16 inside. The upperends of the connecting electrodes 15 and 16 extend to the upper surface2 a of the mount board to establish electrical connections with theelectrode pads 11 and 12.

The lower ends of the connecting electrodes 15 and 16 extend to thebottom surface 2 b of the mount board 2 to establish electricalconnections with terminal electrodes 17 and 18 disposed on the bottomsurface 2 b.

The electrode pads 11 and 12, connecting electrodes 15 and 16, andterminal electrodes 17 and 18 of the mount board 2 are preferably madeof an appropriate metal, such as Al or Cu, or other suitable material.The mount board 2 is made of an insulating ceramic, such as alumina, aninsulating resin, or other suitable material.

The surface acoustic wave element 3 is mounted on the mount board 2, asdescribed above, with a space therebetween. This space A ensures theoscillation of the surface acoustic wave element 3 to prevent thedegradation of the characteristics. The space A is sealed with thesealing resin 4. The sealing resin 4 is bonded to the upper surface 2 aof the mount board 2, with contacts with the upper surface and sides ofthe surface acoustic wave element 3.

The surface acoustic wave device 1 includes the protective film 9 of thesurface acoustic wave element 3. The protective film 9 is intended toprevent the piezoelectric substrate 5 from reproducing the pyroelectriceffect in steps performed in a high-temperature atmosphere containingoxygen during the manufacturing process of the surface acoustic wavedevice 1. The protective film 9 can be formed of any material as long asit acts as intended. Examples of the material of the protective film 9include SiN, ZnO, and SiO₂. Preferably, SiN is used because it does notcontain oxygen. A SiO₂ or ZnO protective film 9 can of course preventreoxidation of the piezoelectric substrate 5.

The protective film 9 in the present preferred embodiment covers theregion where the interdigital electrodes 6 are formed, as shown in FIG.1B. Therefore the surface of the piezoelectric substrate 5 is notbrought into contact with the atmosphere, in the region where theinterdigital electrodes 6 are located. Consequently, the piezoelectricsubstrate 5 in this region can be prevented from reoxidizing.

The steps that are performed in a high-temperature atmosphere containingoxygen and may cause the substrate to reoxidize are subsequent to theformation of the surface acoustic wave element 3, and such steps includebonding the surface acoustic wave element 3 to the mount board 2 by aface-down technique and thermally curing the sealing resin 4, asdescribed above. Although these steps are performed in ahigh-temperature atmosphere containing oxygen, the region covered withthe protective film 9 of the piezoelectric substrate 5 is not reoxidizedin the present preferred embodiment because of the presence of theprotective film 9. Accordingly, the electrodes are not easily degradedor broken by the pyroelectric effect of the surface acoustic wave device1 or the application of surge voltage. This will be further describedwith reference to experiments.

LiTaO₃ substrates having a specific resistance of about 1.0×10⁷ Ω·cm toabout 1.0×10¹³ Ω·cm were prepared.

Some of these piezoelectric substrates were provided with a protectivefilm. The piezoelectric substrates having no protective film and thepiezoelectric substrates having the protective film were heated atrespective temperatures for one hour in air. After sufficient time hadelapsed, the substrates were each measured for voltage induced at theirsurfaces by heating to about 100° C. with a heater. The samepiezoelectric substrates were heated at respective temperatures in anitrogen atmosphere in the same manner as described above. Aftersufficient time had elapsed, the substrates were each measured forvoltage induced at their surfaces by heating to about 100° C. with aheater. In FIG. 2, the circle represents the voltage induced by heatinga LiTaO₃ substrate with a specific resistance of about 10¹⁴ Ω to about100° C.

FIG. 2 shows that, even in the LiTaO₃ substrates having a low specificresistance of about 1.0×10⁷ Ω·cm to about 1.0×10¹³ Ω·cm, which do nothave the pyroelectric effect, the voltage induced at the surface wasincreased by heating to high temperatures in air. This means that thesubstrates can be reoxidized to reproduce a pyroelectric effect. Incontrast, when the substrates were heated in a nitrogen atmosphere, orwhen the substrates having the protective film were heated in anatmosphere containing oxygen, the pyroelectric effect was notreproduced.

In other words, if a piezoelectric substrate with a reduced specificresistance is used for a surface acoustic wave element and a protectivefilm 9 is provided according to present preferred embodiment, thepiezoelectric substrate 5 is prevented from reoxidizing even though itis exposed to a high-temperature atmosphere containing oxygen, as in thecase of heating in a nitrogen atmosphere shown in FIG. 2. Hence, in thepresent preferred embodiment, the protective film 9 makes it difficultto reoxidize the surface of the region covered with the protective film9 of the piezoelectric substrate 5, even though a step or treatment isperformed in a high-temperature atmosphere containing oxygen in amanufacturing process of the surface acoustic wave device 1. Thus, thereproduction of the pyroelectric effect is prevented, and the electrodesare prevented from deteriorating or breaking.

In addition to preventing the reproduction of the pyroelectric effect,the protective film 9 can prevent the IDT electrodes 6 from oxidizingand corroding.

For screening for defective devices in a manufacturing process ofsurface acoustic wave devices using a piezoelectric substrate having apyroelectric effect, a method is generally applied which detects metalpowder resulting in failure, trapped between the fingers of the IDTelectrodes. Specifically, a heat load is applied to induce pyroelectriccharges in a final step so that electric discharge occurs between theelectrode fingers and the metal powder to establish a short circuitbetween the electrode fingers of the electrodes. Thus, surface acousticwave elements on which the metal powder is trapped are separated.

As for piezoelectric substrates whose specific resistances have beenreduced by oxygen reduction, it is generally difficult to apply thismethod for screening, because their pyroelectric effects are low.

In the present preferred embodiment, however, the above-describedscreening step is not necessary. The protective film 9 formed over theIDT electrodes prevents the attachment of metal powder between thefingers of the electrodes. As a result, short-circuiting due to themetal powder between the fingers of the IDT electrodes can be prevented.

FIG. 3A is a schematic front sectional view of a surface acoustic wavedevice according to a second preferred embodiment of the presentinvention. In the surface acoustic wave device 1 of the first preferredembodiment, the protective film 9 is disposed over the region where theinterdigital electrodes 6 are disposed on the main surface 5 a of thepiezoelectric substrate 4. In a surface acoustic wave device 21according to the second preferred embodiment, the protective film 29 ofthe surface acoustic wave element 3 is provided not only over the regionhaving the interdigital electrodes 6, but also over substantially theentire main surface 5 a of the piezoelectric substrate 5. However, theupper surfaces of the electrode pads 7 and 8 are at least partiallyexposed, without being covered with the protective film 29, to establishelectrical contacts with the metal bumps 13 and 14.

The other parts of the surface acoustic wave device 21 are formed in thesame manner as in the surface acoustic wave device 1 of the firstpreferred embodiment. The same parts are designated by the same numeralsand the description is not repeated.

In the second preferred embodiment, since the protective film 29 isformed over substantially the entire surface of the main surface 5 a ofthe piezoelectric substrate 5, the protective film 29 can prevent thereoxidation of the piezoelectric substrate 5 more effectively and,accordingly, enhance the effect of preventing the short circuiting andcorrosion of the electrodes. In the formation of the protective film 29,for example, the film 29 is preferably provided over the entire surfaceof the main surface 5 a of the piezoelectric substrate 5, and is thenpartially etched for forming the electrode pads 7, 8, E1, E2, and E3. Inthis instance, the etched areas or the areas of the exposed portions 29a are preferably smaller than the areas of the electrode pads 7, 8, E1,E2, and E3, as shown in the plan view of the surface acoustic waveelement 3, FIG. 3B. More specifically, it is preferable that theperipheries of the exposed portions 29 a formed by etching lie insidethe peripheries of the electrode pads 7, 8, E1, E2, and E3 so that themain surface 5 a of the piezoelectric substrate 5 is not exposed. Thisprevents an intermetallic compound produced between the metal bumps 13and 14 and the respective electrode pads 7 and 8 from spreading over theentire surfaces of the electrode pads 7 and 8 after the formation of themetal bumps 13 and 14. Since the area of the intermetallic compound isprevented from spreading, the peripheries of the electrode pads are notnegatively affected by the intermetallic compound. Consequently, metalpowder resulting from the degradation of the peripheries of theelectrode pats can be prevented.

FIG. 4 is a schematic front sectional view of a surface acoustic wavedevice according to a third preferred embodiment of the presentinvention. In this surface acoustic wave device 31, a protective film 39is formed directly on the main surface 5 a of the piezoelectricsubstrate 5 as a base layer for the interdigital electrodes 6. Otherparts of the surface acoustic wave device 31 are formed in the samemanner as in the surface acoustic wave device 1 of the first preferredembodiment, except for the protective film 39 serving as the base layerof the interdigital electrodes 6.

The protective film 39 serves as a base layer for the interdigitalelectrodes 6 in the present preferred embodiment. The protective film 39prevents the region having the protective film of the piezoelectricsubstrate 5 from being reoxidized in a high-temperature atmospherecontaining oxygen in the manufacturing process of the surface acousticwave device 1, as in the first preferred embodiment. As a result, theinterdigital electrodes 6 are not easily degraded or broken by thepyroelectric effect, as in the first preferred embodiment.

If the interdigital electrodes 6 have a multilayer structure formed bydepositing an Al layer on a Ti layer in a structure having theprotective film 39, the protective film 39 prevents the Ti layer fromtaking oxygen from the piezoelectric substrate 5. Thus, the protectivefilm 39 can enhance the electrical stability of interdigital electrodeshaving such a multilayer structure.

While the protective film 39 shown FIG. 4 is formed as a base layer inthe region where the interdigital electrodes 6 are disposed, theprotective film 39 may be formed over the entire main surface 5 a of thepiezoelectric substrate 5, as shown in FIG. 9. In this instance, theprotective film 39 also serves as a base layer for the electrode pads 7and 8, unlike the protective film of the second preferred embodiment.The protective film 39 covering the entire main surface 5 a can preventthe reoxidation of the piezoelectric substrate 5 more effectively.

FIG. 5 is a schematic front sectional view of a surface acoustic wavedevice according to a fourth preferred embodiment of the presentinvention. This surface acoustic wave device 41 has a multilayerprotective film 49. The other parts of the surface acoustic wave device41 are formed in the same manner as in the surface acoustic wave device1 of the first preferred embodiment. The protective film 49 includes afirst layer 49 a formed so as to cover the region having theinterdigital electrodes 6 and a second layer 49 b formed over the firstlayer 49 a. The protective film may have such a multilayer structureformed by depositing a plurality of materials. Preferably, the firstlayer 49 a is formed of SiN or ZnO and the second layer 49 b is formedof SiO₂, for example. A SiN layer has superior oxidation resistance andcorrosion resistance even if it is thin. A SiO₂ layer has a higherstability over a longer period of time than the SiN layer. By forming aSiO₂ second layer 49 b over a SiN or ZnO first layer, the resultingprotective film can stably exert its effect over a long time.Furthermore, the thickness of the multilayer protective film 49 is easyto control, and accordingly, the frequency can be easily controlled byvarying the thickness of the protective film. More specifically, whenthe frequency of the surface acoustic wave element 3 is controlled byvarying the thickness of the protective film, the second layer 49 b ispreferably formed of a material having a wider range of choices inetching conditions and being capable of easily controlling thethickness. For example, a SiO₂ layer has a wider range of choices inetching conditions than a SiN layer and the thickness of the SiO₂ layeris easier to control than that of a SiN layer. It is thereforepreferable that the second layer 49 b be formed of SiO₂.

While the protective film 49 shown in FIG. 5 is provided only in theregion where the interdigital electrodes 6 are disposed, the protectivefilm 49 may be formed over substantially the entire main surface 5 a ofthe piezoelectric substrate 5, as in the surface acoustic wave device 21of the second preferred embodiment. In this instance, the first layer 49a or the second layer 49 b may spread over substantially the entire mainsurface.

FIG. 6 is a schematic front sectional view of a surface acoustic wavedevice according to a fifth preferred embodiment of the presentinvention. In this surface acoustic wave device 51, the interdigitalelectrodes 6 are disposed on the protective film 39, as in the surfaceacoustic wave device 31 of the third preferred embodiment. In addition,the surface acoustic wave device 51 has a second protective film 59arranged to cover the interdigital electrodes 6. In other words, theinterdigital electrodes 6 are disposed in a composite of the protectivefilm 39 and the second protective film 59.

The other parts of the surface acoustic wave device 51 are formed in thesame manner as in the surface acoustic wave device 31 of the thirdpreferred embodiment.

The interdigital electrodes 6 may be buried in a composite of theprotective films as in the surface acoustic wave device 51. In thisinstance as well, the protective films 39 and 59 prevent the surface ofthe piezoelectric substrate 5 from being reoxidized by a subsequent stepperformed in a high-temperature atmosphere containing oxygen. Inaddition, the second protective film 59 can prevent the oxidation andcorrosion of the interdigital electrodes 6.

In the present preferred embodiment as well, either of the protectivefilms 39 and 59 may spread over substantially the entire main surface 5a of the piezoelectric substrate 5. However, the protective films 39 and59 must be formed in a region other than regions where the electrodepads 7 and 8 are formed. In these regions, the electrodes pads 7 and 8need to establish electrical connections with metal bumps 13 and 14, orother suitable connection elements. The protective film 39 may be formedas a base layer for the electrode pads 7 and 8, as shown in FIG. 10. Inthis instance, the protective film 39 may spread over the entire mainsurface 5 a of the piezoelectric substrate 5.

FIG. 7 is a schematic front sectional view of a surface acoustic wavedevice according to a sixth preferred embodiment of the presentinvention. This surface acoustic wave device 61 has the same surfaceacoustic wave element 3 as in the surface acoustic wave device 1 of thefirst preferred embodiment. In the present preferred embodiment, thepackage has a different structure from the package of the surfaceacoustic wave device 1 of the first preferred embodiment. Morespecifically, the surface acoustic wave device 61 of the presentinvention uses a package 62 having a recess 62 a that is arranged toopen upward. The surface acoustic wave element 3 is housed in the recess62 a of the package 62. The bottom 62 b of the recess 62 a is providedwith the electrode pads 11 and 12 on its surface. In addition, thepackage 62 has the connecting electrodes 15 and 16 and the terminalelectrodes 17 and 18 in the same manner as the mount board 2 (see FIGS.1A and 1B). Then, a cover 63 is bonded to the package 62 with aninsulative adhesive or the like so as to close the recess 62 a. Thepackage 62 is made of an insulating ceramic as the mount board 2 is. Thecover 63 is made of a metal or an insulating material. A metal cover 63can shield the upper side of the recess 62 a from electromagnetism.Furthermore, the presence of the protective film 9 prevents thepiezoelectric substrate 5 from being oxidized by face-down bonding.

FIG. 8 is a schematic front sectional view of a surface acoustic wavedevice according to a seventh preferred embodiment of the presentinvention. In this preferred embodiment, the surface acoustic waveelement 3 is housed in a recess 72 a of a package 72. In this instance,the surface acoustic wave element 3 is fixed to the package 72 at theother main surface 5 b of the piezoelectric substrate 5 with aninsulating adhesive 73, but not by face-down bonding.

More specifically, the surface acoustic wave element 3 is housed andfixed in the recess 72 a in such a manner that the main surface havingthe interdigital electrodes 6 faces upward.

The recess 72 a of the package 72 has a step 72 c on which electrodepads 74 and 75 are disposed. The electrode pads 74 and 75 are connectedto the electrode pads 7 and 8 of the surface acoustic wave element 3with bonding wires 76 and 77.

The package 72 has connecting electrodes 80 and 81 to connect theelectrode pads 74 and 75 to terminal electrodes 78 and 79 formed on thebottom surface of the package 72.

A cover 82 is fixed to the package 72 to close the recess 72 a. Thecover 82 is made of the same material as the cover 63 of the surfaceacoustic wave device 61 of the sixth preferred embodiment. The presenceof the protective film 9 prevents the piezoelectric substrate 5 frombeing oxidized by die bonding with an insulating adhesive.

As the surface acoustic wave devices 61 and 71 of the sixth and seventhpreferred embodiments show, the surface acoustic wave element of thepresent invention can be used in surface acoustic wave devices havingvarious types of packages. The structure of the package is notparticularly limited.

The electrodes of the surface acoustic wave element of the presentinvention are not limited to the structure shown in FIG. 1B, and thepresent invention can be applied to surface acoustic wave elementshaving various electrode structures, such as those used in surfaceacoustic wave resonators and surface acoustic wave filters.

The surface acoustic wave element according to the first preferredembodiment preferably includes a piezoelectric substrate that has a lowspecific resistance of about 1.0×10⁷ Ω·cm to about 1.0×10¹³ Ω·cm and,accordingly, does not produce a pyroelectric effect. In addition, aprotective film is arranged so as to cover at least the interdigitalelectrodes. The protective film can prevent the piezoelectric substratefrom being reoxidized by a step performed in a high-temperatureatmosphere containing oxygen in a manufacturing process of a surfaceacoustic wave device including the surface acoustic wave element. Sincethe piezoelectric substrate is not easily reoxidized even if the surfaceacoustic wave element is exposed to a high-temperature atmospherecontaining oxygen, the interdigital electrodes can be prevented frombeing degraded or broken, effectively.

Furthermore, the protective film covers the interdigital electrodes toprevent the oxidation and corrosion of the interdigital electrodes.

In the second preferred embodiment as well, the surface acoustic waveelement uses a piezoelectric substrate having a low specific resistanceof about 1.0×10⁷ Ω·cm to about 1.0×10¹³ Ω·cm and has a protective filmas a base layer for the interdigital electrodes. The protective filmprevents the reoxidation of the piezoelectric substrate. Consequently,the piezoelectric substrate in the second preferred embodiment isdifficult to reoxidize even if it is exposed to a high-temperatureatmosphere containing oxygen. Accordingly, the degradation or breakageof the electrodes resulting from the pyroelectric effect of thepiezoelectric substrate does not easily occur.

In the second preferred embodiment, if a second protective film isarranged so as to cover the interdigital electrodes, the secondprotective film not only prevents the reoxidation of the piezoelectricsubstrate, but also protects the interdigital electrodes. Thus, theoxidation resistance and corrosion resistance of the interdigitalelectrodes can be enhanced as in the first preferred embodiment.

The protective film may be formed of various materials. A protectivefilm formed of one material selected from among SiN, ZnO, and SiO₂ canprevent the reoxidation of the piezoelectric substrate effectively.

The protective film may have a multilayer structure. If the protectivefilm is formed by depositing a SiO₂ layer on a SiN or ZnO layerprotective film, its thickness can be reduced because the SiN or ZnOlayer has superior oxidation resistance and corrosion resistance even ifits thickness is small. In addition, the SiO₂ layer has a wider range ofchoices in etching conditions than the SiN or ZnO layer and thethickness of the SiO₂ layer is easier to control than that of the SiN orZnO layer. By forming a SiO₂ layer as the second layer of the protectivefilm, the frequency can be easily controlled with high precision.

For screening defectives, it is tested whether a short circuit isestablished between metal powder and the fingers of the electrodes.However, if the number of short circuits is one, the short circuitcannot be detected, and screening is not correctly carried out. Theprotective film prevents the attachment of metal powder on theelectrodes.

In various preferred embodiments of the present invention, if a LiTaO₃or LiNbO₃ substrate is used as the piezoelectric substrate, the specificresistance of the piezoelectric substrate can be easily reduced to about1.0×10⁷ Ω·cm to about 1.0×10¹³ Ω·cm by heating at a high temperatureunder reduced pressure.

If the protective film is formed over substantially an entire mainsurface of the piezoelectric substrate on which electrode pads areformed, such that the protective film has regions at which the electrodepads are exposed with an area smaller than that of the electrode pads,the protective film can certainly prevent the reoxidation of thepiezoelectric substrate.

While the present invention has been described with respect to preferredembodiments, it will be apparent to those skilled in the art that thedisclosed invention may be modified in numerous ways and may assume manyembodiments other than those specifically set out and described above.Accordingly, it is intended by the appended claims to cover allmodifications of the invention that fall within the true spirit andscope of the invention.

1-7. (canceled)
 8. A surface acoustic wave element comprising: apiezoelectric substrate having a specific resistance in a range of about1.0×10⁷ Ω·cm to about 1.0×10¹³ Ω·cm; an interdigital electrode disposedon a main surface of the piezoelectric substrate; and a protective filmcovering the interdigital electrode.
 9. The surface acoustic waveelement according to claim 8, wherein the protective film includes amaterial selected from the group consisting of SiN, ZnO, and SiO₂. 10.The surface acoustic wave element according to claim 8, wherein theprotective film includes a SiN or ZnO layer and a SiO₂ layer depositedon the SiN or ZnO layer.
 11. The surface acoustic wave element accordingto claim 8, wherein the piezoelectric substrate includes LiTaO₃ orLiNbO₃.
 12. The surface acoustic wave element according to claim 8,further comprising an electrode pad for establishing external electricalconnection, disposed on the piezoelectric substrate, wherein theprotective film is arranged so as to have a region at which theelectrode pad is exposed, and the region has an area that is smallerthan the area of the electrode pad.
 13. The surface acoustic waveelement according to claim 8, further comprising a plurality ofinterdigital electrodes disposed on the main surface, wherein theprotective film is arranged to cover the plurality of interdigitalelectrodes.
 14. The surface acoustic wave element according to claim 8,wherein the protective film is arranged to cover substantially theentire main surface of the piezoelectric substrate.
 15. The surfaceacoustic wave element according to claim 8, wherein the protective filmis a multilayer film.
 16. A surface acoustic wave device comprising: amount board; and the surface acoustic wave element according to claim 8;wherein the surface acoustic wave element is mounted on the mount boardwith the main surface facing downward with a space being providedbetween the surface acoustic wave element and the mount board and thesurface acoustic wave element being sealed to the mount board.
 17. Asurface acoustic wave device comprising: a package having a recess; thesurface acoustic wave element according to claim 8 housed in the recessof the package; and a cover bonded to the package.
 18. A surfaceacoustic wave element comprising: a piezoelectric substrate having aspecific resistance in the range of about 1.0×10⁷ Ω·cm to about 1.0×10¹³Ω·cm; an interdigital electrode overlying a main surface of thepiezoelectric substrate; and a protective film underlying theinterdigital electrode.
 19. The surface acoustic wave element accordingto claim 18, further comprising a second protective film covering theinterdigital electrode.
 20. The surface acoustic wave element accordingto claim 18, wherein the protective film includes a material selectedfrom the group consisting of SiN, ZnO, and SiO₂.
 21. The surfaceacoustic wave element according to claim 18, wherein the protective filmincludes a SiN or ZnO layer and a SiO₂ layer deposited on the SiN or ZnOlayer.
 22. The surface acoustic wave element according to claim 18,wherein the piezoelectric substrate includes LiTaO₃ or LiNbO₃.
 23. Thesurface acoustic wave element according to claim 18, further comprisingan electrode pad for establishing external electrical connection,disposed on the piezoelectric substrate, wherein the protective film isarranged so as to have a region at which the electrode pad is exposed,and the region has an area that is smaller than the area of theelectrode pad.
 24. The surface acoustic wave element according to claim18, further comprising a plurality of interdigital electrodes disposedon the main surface, wherein the protective film is arranged to coverthe plurality of interdigital electrodes.
 25. The surface acoustic waveelement according to claim 18, wherein the protective film is arrangedto cover substantially the entire main surface of the piezoelectricsubstrate.
 26. The surface acoustic wave element according to claim 18,wherein the protective film is a multilayer film.
 27. A surface acousticwave device comprising: a mount board; and the surface acoustic waveelement according to claim 18; wherein the surface acoustic wave elementis mounted on the mount board with the main surface facing downward witha space being provided between the surface acoustic wave element and themount board and the surface acoustic wave element being sealed to themount board.
 28. A surface acoustic wave device comprising: a packagehaving a recess; the surface acoustic wave element according to claim 18housed in the recess of the package; and a cover bonded to the package.